Discrete proximity detection system

ABSTRACT

A machine is described having at least one dangerous portion. The machine also includes a safety system to detect dangerous proximity between a person and the dangerous portion and perform a predetermined action to limit or prevent injury to the person from the dangerous portion.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of and priority from U.S.Provisional Patent Application Serial No. 60/302,937, filed Jul. 2,2001, which is hereby incorporated by reference.

FIELD

[0002] The present invention relates to systems for discretely detectingthe proximity of a human to sensor and, in more specific embodiments,for discretely detecting the proximity of a human to a dangerous portionof a piece of power equipment.

BACKGROUND

[0003] Safety systems often must be able to detect the presence of ahuman body or some portion thereof in a dangerous location. Forinstance, interruption of a beam in a light curtain is used to sense thepresence of an operator near a dangerous machine. When the beam isbroken, the machine can be shut down automatically to reduce the chanceof injury. Light curtains, however, are limited to protecting areas thatcan be bounded by planar surfaces, and that will not be penetrated byany object, not just humans. However, there are many instances whereworkpieces must enter an area that cannot be protected by a lightcurtain because the light curtain cannot distinguish between a human anda workpiece.

[0004] As a solution to the problem of distinguishing work pieces fromhumans, numerous radio frequency proximity detection systems have beendeveloped. The radio-frequency systems create a radio-frequency electricfield in a protected area that is altered when some part of a human bodyenters the area. A limitation with such systems is that they are notvery discrete in detection. More specifically, it is very difficult toprecisely define the protection area with radio-frequency systems.Typically, the system cannot determine the distance to the bodyaccurately and therefore a hand or body at long range may generate thesame signal change as a finger in closer proximity. As a result,radio-frequency systems are prone to false trips or may miss actualdangerous conditions.

[0005] In view of the limitations of the prior art detection systems, itwould be desirable to have a system that could reliably and discretelydetect proximity between a user and a dangerous location prior to actualcontact.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a schematic block diagram of a machine with afast-acting safety system.

[0007]FIG. 2 is a schematic diagram of an exemplary safety system in thecontext of a machine having a circular blade, and configured to detectdangerous proximity between a person and the blade.

[0008]FIG. 3 is a schematic partial side view of a machine having a highvoltage generation system in the form of a drive belt coupling between amotor and a blade.

[0009]FIG. 4 is a schematic diagram of an alternative high voltagegeneration system.

DETAILED DESCRIPTION

[0010] A machine according to the present invention is shownschematically in FIG. 1 and indicated generally at 10. Machine 10 may beany of a variety of different types and configurations of machineadapted for cutting workpieces, such as wood, plastic, etc. Machine 10includes an operative structure 12 having a cutting tool 14 and a motorassembly 16 adapted to drive the cutting tool. Machine 10 also includesa safety system 18 configured to minimize the potential of a seriousinjury to a person using machine 10. Safety system 18 is adapted todetect the occurrence of one or more dangerous, or triggering,conditions during use of machine 10. If such a dangerous condition isdetected, safety system 18 is adapted to engage operative structure 12to limit any injury to the user caused by the dangerous condition.

[0011] Machine 10 also includes a suitable power source 20 to providepower to operative structure 12 and safety system 18. Power source 20may be an external power source such as line current, or an internalpower source such as a battery. Alternatively, power source 20 mayinclude a combination of both external and internal power sources.Furthermore, power source 20 may include two or more separate powersources, each adapted to power different portions of machine 10.

[0012] It will be appreciated that operative structure 12 may take anyone of many different forms, depending on the type of machine 10. Forexample, operative structure 12 may include a stationary housingconfigured to support motor assembly 16 in driving engagement withcutting tool 14. Alternatively, operative structure 12 may include amovable structure configured to carry cutting tool 14 between multipleoperating positions. As a further alternative, operative structure 12may include one or more transport mechanisms adapted to convey aworkpiece toward and/or away from cutting tool 14. In the case of chopsaws, operative structure 12 typically takes the form of an arbor blockpivotally coupled to a base or frame. Cutting tool 14 is mounted on thearm and pivotal upward toward a workpiece supported by the base.

[0013] Safety system 18 includes a detection subsystem 22, a reactionsubsystem 24 and a control subsystem 26. Control subsystem 26 may beadapted to receive inputs from a variety of sources including detectionsubsystem 22, reaction subsystem 24, operative structure 12 and motorassembly 16. The control subsystem may also include one or more sensorsadapted to monitor selected parameters of machine 10. In addition,control subsystem 26 typically includes one or more instruments operableby a user to control the machine. The control subsystem is configured tocontrol machine 10 in response to the inputs it receives.

[0014] Detection subsystem 22 is configured to detect one or moredangerous, or triggering, conditions during use of machine 10. Forexample, the detection subsystem may be configured to detect that aportion of the user's body is dangerously close to, or in contact with,a portion of cutting tool 14. As another example, the detectionsubsystem may be configured to detect the rapid movement of a workpiecedue to kickback by the cutting tool, as is described in U.S. ProvisionalPatent Application Serial No. 60/182,866, filed Feb. 16, 2000, and U.S.Pat. No. 4,267,914, the disclosures of which are herein incorporated byreference. In some embodiments, detection subsystem 22 may informcontrol subsystem 26 of the dangerous condition, which then activatesreaction subsystem 24. In other embodiments, the detection subsystem maybe adapted to activate the reaction subsystem directly.

[0015] Once activated in response to a dangerous condition, reactionsubsystem 24 is configured to engage operative structure 12 quickly toprevent serious injury to the user. It will be appreciated that theparticular action to be taken by reaction subsystem 24 will varydepending on the type of machine 10 and/or the dangerous condition thatis detected. For example, reaction subsystem 24 may be configured to doone or more of the following: stop the movement of cutting tool 14,disconnect motor assembly 16 from power source 20, place a barrierbetween the cutting tool and the user, retract the cutting tool from itsoperating position, etc. The reaction subsystem may be configured totake a combination of steps to protect the user from serious injury.Placement of a barrier between the cutting tool and teeth is describedin more detail in U.S. Provisional Patent Application Serial No.60/225,206, filed Aug. 14, 2000, the disclosure of which is hereinincorporated by reference. Retraction of the cutting tool from itsoperating position is described in more detail in U.S. ProvisionalPatent Application Serial No. 60/225,089, filed Aug. 14, 2000, thedisclosure of which is herein incorporated by reference.

[0016] The configuration of reaction subsystem 24 typically will varydepending on which action(s) are taken. In the exemplary embodimentdepicted in FIG. 1, reaction subsystem 24 is configured to stop themovement of cutting tool 14 and includes a brake mechanism 28, a biasingmechanism 30, a restraining mechanism 32, and a release mechanism 34.Brake mechanism 28 is adapted to engage operative structure 12 under theurging of biasing mechanism 30. During normal operation of machine 10,restraining mechanism 32 holds the brake mechanism out of engagementwith the operative structure. However, upon receipt of an activationsignal by reaction subsystem 24, the brake mechanism is released fromthe restraining mechanism by release mechanism 34, whereupon, the brakemechanism quickly engages at least a portion of the operative structureto bring the cutting tool to a stop.

[0017] It will be appreciated by those of skill in the art that theexemplary embodiment depicted in FIG. 1 and described above may beimplemented in a variety of ways depending on the type and configurationof operative structure 12. Turning attention to FIG. 2, one example ofthe many possible implementations of machine 10 includes a cutting tool14 in the form of a circular blade 40 mounted on a rotating shaft orarbor 42. Blade 40 includes a plurality of cutting teeth (not shown)disposed around the outer edge of the blade. As described in more detailbelow, brake mechanism 28 is adapted to engage the teeth of blade 40 andstop rotation of the blade.

[0018] In the exemplary implementation, detection subsystem 22 isadapted to detect the dangerous condition of the user coming into closeproximity to blade 40. The detection subsystem operates by impartingthrough a connection 44 a high voltage charge to blade 40 and watchingfor the electrical discharge that will occur when the user's handapproaches the blade closely enough that the electric field between theuser and the blade will exceed the dielectric breakdown strength of theair separating the blade and the user.

[0019] Typically, the blade, or some larger portion of cutting tool 14,is electrically isolated from the remainder of machine 10 to permitcreation of the electrical charge on the blade. Alternatively, detectionsubsystem 22 may include a different sensor assembly configured todetect proximity in other ways, such as optically, resistively, etc. Inany event, the detection subsystem is adapted to transmit a signal tocontrol subsystem 26 when dangerous proximity between the user and theblade is detected. Various exemplary embodiments and implementations ofalternative detection subsystem 22 are described in more detail in U.S.Provisional Patent Application Serial No. 60/225,200, filed Aug. 14,2000, U.S. Provisional Patent Application Serial No. 60/225,211, filedAug. 14, 2000, and U.S. Provisional Patent Application Serial No.60/270,011, filed Feb. 20, 2001, the disclosures of which are hereinincorporated by reference.

[0020] Control subsystem 26 includes one or more instruments 48 that areoperable by a user to control the motion of blade 40. Instruments 48 mayinclude start/stop switches, speed controls, direction controls, etc.Control subsystem 26 also includes a logic controller 50 connected toreceive the user's inputs via instruments 48. Logic controller 50 isalso connected to receive a detection signal from detection subsystem22. Further, the logic controller may be configured to receive inputsfrom other sources (not shown) such as blade motion sensors, workpiecesensors, etc. In any event, the logic controller is configured tocontrol operative structure 12 in response to the user's inputs throughinstruments 48. However, upon receipt of a detection signal fromdetection subsystem 22, the logic controller overrides the controlinputs from the user and activates reaction subsystem 24 to stop themotion of the blade. Various exemplary embodiments and implementationsof control subsystem 26 are described in more detail in U.S. ProvisionalPatent Application Serial No. 60/225,059, filed Aug. 14, 2000 and U.S.Provisional Patent Application Serial No. 60/225,094, filed Aug. 14,2000, the disclosures of which are herein incorporated by reference.

[0021] In the exemplary implementation shown in FIG. 2, brake mechanism28 includes a pawl 60 mounted adjacent the edge of blade 40 andselectively moveable to engage and grip the teeth of the blade. Pawl 60may be constructed of any suitable material adapted to engage and stopthe blade. As one example, the pawl may be constructed of a relativelyhigh strength thermoplastic material such as polycarbonate, ultrahighmolecular weight polyethylene (UHMW), Acrylonitrile Butadiene Styrene(ABS), etc., or a metal such as aluminum, etc. It will be appreciatedthat the construction of pawl 60 will vary depending on theconfiguration of blade 40. In any event, the pawl is urged into theblade by a biasing mechanism such as a spring 66. In the illustrativeembodiment shown in FIG. 2, pawl 60 is pivoted into the teeth of blade40. It should be understood that sliding or rotary movement of pawl 60may also be used. The spring is adapted to urge pawl 60 into the teethof the blade with sufficient force to grip the blade and quickly bringit to a stop.

[0022] The pawl is held away from the edge of the blade by a restrainingmechanism such as a fusible member 70. The fusible member is constructedof a suitable material adapted to restrain the pawl against the bias ofspring 66, and also adapted to melt under a determined electricalcurrent density. Examples of suitable materials for fusible member 70include NiChrome wire, stainless steel wire, etc. The fusible member isconnected between the pawl and a contact mount 72. Preferably, fusiblemember 70 holds the pawl relatively close to the edge of the blade toreduce the distance pawl 60 must travel to engage blade 40. Positioningthe pawl relatively close to the edge of the blade reduces the timerequired for the pawl to engage and stop the blade. Typically, the pawlis held approximately {fraction (1/32)}-inch to ¼-inch from the edge ofthe blade by fusible member 70; however other pawl-to-blade spacings mayalso be used within the scope of the invention.

[0023] Pawl 60 is released from its unactuated, or cocked, position toengage blade 40 by a release mechanism in the form of a firing subsystem76. The firing subsystem is coupled to contact mount 72, and isconfigured to melt fusible member 70 by passing a surge of electricalcurrent through the fusible member. Firing subsystem 76 is coupled tologic controller 50 and activated by a signal from the logic controller.When the logic controller receives a detection signal from detectionsubsystem 22, the logic controller sends an activation signal to firingsubsystem 76, which melts fusible member 70, thereby releasing the pawlto stop the blade. Various exemplary embodiments and implementations ofreaction subsystem 24 are described in more detail in U.S. ProvisionalPatent Application Serial No. 60/225,056, filed Aug. 14, 2000, U.S.Provisional Patent Application Serial No. 60/225,169, filed Aug. 14,2000, and U.S. Provisional Patent Application Serial No. 60/225,170,filed Aug. 14, 2000, the disclosures of which are herein incorporated byreference.

[0024] It will be appreciated that activation of the brake mechanism mayrequire the replacement of one or more portions of safety system 18. Forexample, pawl 60 and fusible member 70 typically are single-usecomponents which must be replaced before the safety system is ready tobe used again. Thus, it may be desirable to incorporate one or moreportions of safety system 18 in a cartridge that can be easily replaced.For example, in the exemplary implementation depicted in FIG. 2, safetysystem 18 includes a replaceable cartridge 80 having a housing 82. Pawl60, spring 66, fusible member 70 and contact mount 72 are all mountedwithin housing 82. Alternatively, other portions of safety system 18 maybe mounted within the housing. In any event, after the reactionsubsystem has been activated, the safety system can be reset byreplacing cartridge 80. The portions of safety system 18 not mountedwithin the cartridge may be replaced separately or reused asappropriate. Various exemplary embodiments and implementations of asafety system using a replaceable cartridge are described in more detailin U.S. Provisional Patent Application Serial No. 60/225,201, filed Aug.14, 2000 and U.S. Provisional Patent Application Serial No. 60/225,212,filed Aug. 14, 2000, the disclosures of which are herein incorporated byreference.

[0025] In the case of miter saws, chop saws, radial arm saws, and otherpower equipment in which a cutting tool moves toward and into aworkpiece to cut the workpiece, reaction subsystem 24 may include asystem to stop the cutting tool from continuing to move into theworkpiece. Stopping the translational motion of the cutting tool canprevent or minimize any injury from accidental contact between a userand the cutting tool.

[0026] Connection 44 may take many different forms. For example,connection 44 may take the form of a conductive slip ring or conductivebrush in direct contact with the blade, or some member electricallycoupled to the blade, such as the arbor. It should be noted that theterm blade is used for convenience, but the actual element which is usedas the sensor may be a blade, some other type of cutter, guard, or someother member placed where proximity by a user's body would be indicativeof a dangerous condition. As another example, connection 44 may be arelatively short air gap. As voltage is applied, the charge can jump ashort gap to charge the blade. The gap should preferably besubstantially smaller than the desired detection distance at which theuser's finger or hand should be detected. Typical gaps would be lessthan 1 mm. Connection 44 may also be capacitive as described in U.S.Provisional Application Serial No. 60/225,211, filed Aug. 14, 2000. Inaddition, blade 40 may be electrically isolated in any suitable fashion,such as disclosed in the above-cited case. It should be noted that theisolation of the blade should be sufficient to prevent dielectricbreakdown and resultant arcing at any point between the blade or otherisolated members and grounded portions of the saw.

[0027] Detection subsystem 22 includes a high voltage generation system46 to create a high voltage to be applied to the blade through coupling44. As shown in FIG. 3, the simplest high voltage generation system andcoupling may be an insulated belt 102 running between a motor pulley 104and an arbor pulley 106. With the proper triboelectric selection ofmaterials, this arrangement can act like a Van De Graff generator toimpart a high voltage static charge to blade 40. In this configuration,it may be desirable to have a charge dissipation element 108 connectedto the blade to limit the charge build up on the blade. This might be asurge suppression device, such as a transient voltage suppressor Zenerdiode, to divert charge after a desired voltage has been established onthe blade, or a resistor to dissipate charge at a known rate so that theequilibrium voltage on the blade is at the desired level. Alternatively,a safety gap formed by two closely-spaced electrodes, one associatedwith the blade and the other with ground, may be used to limit the bladevoltage. When the blade voltage exceeds the arc-over distance betweenthe electrodes, arcing will occur and the blade voltage will be reduced.

[0028] In an alternative embodiment illustrated in FIG. 4, high voltagegeneration system 46 incorporates a high voltage power supply 110 togenerate a known voltage. Power supply 110 is coupled to impart a highvoltage signal to blade 40 through a coupling 44. As described above,coupling 44 may be configured to couple the high voltage power supplydirectly to the blade, or indirectly through some other component suchas arbor 42 that is electrically coupled to the blade.

[0029] Power supply 110 may be any of a variety of suitable high voltagepower supplies such as are well known in the art. Examples of suitablepower supplies include those found in electronic ignitions, flybacktransformer supplies such as are used in CRTs, spark gap circuits suchas are used in tazers, etc. Alternatively, power supply 110 may be aregulated high voltage power supply such as a Bertan Model 605C-200P, N0-20 kV 250 microamp power supply or similar power supply. It will beappreciated that the particular supply chosen will depend on theapplication voltage, current and other characteristics required.

[0030] The voltage applied to the blade may be a DC voltage that couldbe applied either continuously or intermittently. Alternatively, thevoltage may be AC and applied continuously or in intermittentalternating sign DC pulses. The intermittent application may be usefulwhere there is a relatively high-resistance grounding path connectingthe blade to a grounded member of the machine. For instance, work piecessuch as wet or green wood may provide a path to ground that coulddischarge a DC voltage from the blade at a greater rate than could besafely maintained at high voltage. By applying a pulsed signal of singleor alternative sign and looking at the peak pulse voltage or behaviornear peak, the effect of discharge through green wood or other parasiticconductive path can be minimized.

[0031] If the voltage is pulsed, the frequency of pulsing is preferablyat least 100 cycles per second (CPS), and more preferably 500-5000 CPS.As the cycle rate goes down, the required detection distance mustincrease to insure that the reaction system can protect the useradequately. For instance, at 100 CPS, the user may approach within thedetection distance for up to 10 ms before a pulse occurs. This mayrepresent a travel distance of 5 mm or more. Thus if the detectiondistance is 3 mm, the user could have already reached the blade, priorto detection. Thus, the cycle rate should be chosen to insure safedetection in view of the expected detection distance and approachvelocity.

[0032] The voltage applied to the blade is selected to create adielectric breakdown between the user and the blade at a desireddistance. The typical dielectric breakdown strength of air is about30,000 volts per centimeter. Thus, if the desired detection distance is5 mm, the voltage applied should be approximately 15,000 volts. Typicalvoltages would range between 300 and 60,000 volts and more preferablybetween 3,000 and 12,000 volts. These ranges will provide detectiondistances between approximately 0.1 mm and approximately 20 mm andbetween approximately 1 and approximately 4 mm, respectively, althoughthe exact breakdown distances are dependent on electrodeshape—finger/hand/body and blade—and air conditions such as humitity anddust contamination. Fortunately, the sharp points on the teeth of a sawblade tend to concentrate the electric field and therefore decrease thevoltage necessary to generate an arc to the user. Depending on theapplication it may be desirable to alternate the sign of the appliedvoltage so that if the user's body has a static charge relative toground, the voltage will be additive at least on one sign of the highvoltage, thereby insuring reliable detection.

[0033] As shown in FIG. 4, detection subsystem 22 also includes a staticdischarge detection system or module 112. The static discharge detectionmodule is coupled to a sensor assembly 114 adapted to detect a staticdischarge between the blade and a human body. Sensor assembly 114 iscoupled to sense the voltage on blade 40 either directly, or indirectlythrough some other component such as arbor 42 that is electricallycoupled to the blade. The coupling may be either conductive orcapacitive. In any event, discharge detection module 112 is configuredto monitor the voltage on the blade and detect any change in voltage dueto a static discharge event. In one exemplary embodiment, the dischargedetection module is configured to monitor the voltage on the blade andcompare the detected voltage to the expected voltage. In other words,the voltage sensed by the discharge detection module is correlated withthe voltage imparted by the high voltage generation system. If thesensed voltage departs from an expected voltage greater than a thresholdamount, a discharge event is detected. It will be appreciated that theexpected voltage will depend on the magnitude, frequency and polarity ofthe applied voltage, and may be a value that varies with time.

[0034] In addition to detecting that a discharge has occurred, it isalso possible to gather information from the sign of the dischargepulse. For instance, depending on the triboelectric properties ofstatic-creating objects in proximity to the machine, including the user,it may be that static discharges to the blade will occur due to eventsother than close proximity of the user. However, in some cases it willbe possible to use the sign of the discharge pulse to distinguish theseevents from human proximity discharges.

[0035] In addition to looking at the sign of the discharge, in somecases it will be possible to distinguish human proximity discharges fromother events by looking at the discharge rate. In particular, theconductivity of the human body will create a discharge rate within apredetermined range. Human Body Model tests typically represent a humanbody as having a resistance of 1500 ohms and a capacitance of 100picofarads for static discharge testing. Other types of parasiticdischarges, such as might occur through wet or green wood, willtypically exhibit different discharge rates. By monitoring the dischargecurrent directly or through monitoring the rate of change of voltage onthe blade, it is possible to distinguish certain types of non-humandischarges from discharges indicative of a dangerous proximity.

[0036] Other properties of the discharge pulse can also be used to morereliably distinguish dangerous conditions from incidental or parasiticdischarges. For instance, the rise time of the current of the dischargepulse may vary with the approach speed of the body to the blade.Therefore, by analyzing the rise rate of the discharge current, undersome circumstances it will be possible to measure the approach speed andtherefore evaluate the danger. In addition, with a relatively highfrequency pulsed voltage, several samples may be taken to furthercharacterize the danger. For instance, subsequent samples may show thatthe hand is no longer approaching the blade or is approaching morerapidly. In such case, the safety system may be configured not to react(or to react differently) if it determines the user's hand is movingaway from the blade.

[0037] While detection subsystem 22 has been described above in thegeneral context of a woodworking machine having a circular saw blade, itwill be appreciated that the detection subsystem described herein may beadapted for use with a wide variety of different machines having avariety of different configurations. Furthermore, detection subsystem 22may be incorporated into a variety of different configurations of safetysystem 18. Examples of such other configurations of machine 10 and/orsafety system 18 are described in the references incorporated above, aswell in the following patent applications which are incorporated hereinby reference: PCT Patent Application Serial No. PCT/US00/26812, filedSep. 29, 2000; U.S. patent application Ser. No. 09/676,190, filed Sep.29, 2000; U.S. Provisional Patent Application Serial No. 60/298,207,filed Jun. 13, 2001; U.S. Provisional Patent Application Serial No.60/292,100, filed May 17, 2001; U.S. Provisional Patent ApplicationSerial No. 60/292,081, filed May 17, 2001; U.S. Provisional PatentApplication Serial No. 60/279,313, filed Mar. 27, 2001; U.S. ProvisionalPatent Application Serial No. 60/275,595, filed Mar. 13, 2001; U.S.Provisional Patent Application Serial No. 60/275,594, filed Mar. 13,2001; U.S. Provisional Patent Application Serial No. 60/273,902, filedMar. 6, 2001; U.S. Provisional Patent Application Serial No. 60/273,178,filed Mar. 2, 2001; U.S. Provisional Patent Application Serial No.60/273,177, filed Mar. 2, 2001; U.S. Provisional Patent ApplicationSerial No. 60/270,942, filed Feb. 22, 2001; U.S. Provisional PatentApplication Serial No. 60/270,941, filed Feb. 22, 2001; U.S. ProvisionalPatent Application Serial No. 60/233,459, filed Sep. 18, 2000; U.S.Provisional Patent Application Serial No. 60/225,210, filed Aug. 14,2000; U.S. Provisional Patent Application Serial No. 60/225,058, filedAug. 14, 2000; U.S. Provisional Patent Application Serial No.60/225,057, filed Aug. 14, 2000; and U.S. Provisional Patent ApplicationSerial No. 60/157,340, filed Oct. 1, 1999.

We claim:
 1. A woodworking machine comprising: at least one dangerousportion; a detection system configured to detect when any portion of aperson's body is at a distance from the dangerous portion that is withina predetermined range of distances; and a reaction system configured toperform one or more predetermined actions in the event the detectionsystem detects that a portion of a person's body is at a distance fromthe dangerous portion that is within the predetermined range ofdistances.
 2. The woodworking machine of claim 1, where the dangerousportion is a cutting tool adapted to cut wood.
 3. The woodworkingmachine of claim 2, where the predetermined range of distances is fromapproximately 0.1 millimeter to approximately 20 millimeters.
 4. Thewoodworking machine of claim 2, where the predetermined range ofdistances is from approximately 1 millimeter to approximately 4millimeters.
 5. The woodworking machine of claim 2, where the detectionsystem includes a high voltage generation subsystem configured togenerate a high voltage, and a connection configured to couple, directlyor indirectly, the high voltage to the cutting tool.
 6. The woodworkingmachine of claim 5, where the high voltage generation subsystem includesa belt movable to generate static charge.
 7. The woodworking machine ofclaim 6, where the detection system includes a charge dissipationelement configured to limit static charge buildup on the cutting tool.8. The woodworking machine of claim 5, where the high voltage generationsubsystem includes a high voltage power supply.
 9. The woodworkingmachine of claim 5, further comprising an arbor configured to supportthe cutting tool, and where the connection is configured to couple thehigh voltage to the cutting tool through the arbor.
 10. The woodworkingmachine of claim 5, further comprising a static discharge detectionsubsystem configured to detect a static discharge between the cuttingtool and a person's body.
 11. For use on power equipment having at leastone dangerous portion, a method of detecting dangerous proximity betweena person and the dangerous element, the method comprising: generating ahigh voltage; applying the high voltage to the dangerous portion, wherethe high voltage is sufficient to cause an electrical discharge betweenthe dangerous portion and the person's body when any portion of theperson's body comes within a predetermined distance from the dangerousportion; and detecting when an electrical discharge between thedangerous portion and the person's body occurs.
 12. The method of claim11, where the power equipment is a woodworking machine and the dangerousportion is a cutting tool.
 13. The method of claim 11, where the step ofgenerating a high voltage includes generating a DC voltage.
 14. Themethod of claim 13, where the step of generating a high voltage includesgenerating an intermittent DC voltage.
 15. The method of claim 11, wherethe step of generating a high voltage includes generating an AC voltage.16. The method of claim 11, where the predetermined distance is at leastapproximately 0.1 millimeter and less than approximately 20 millimeters.17. The method of claim 11, where the predetermined distance is at leastapproximately 1 millimeter and less than approximately 4 millimeters.18. A woodworking machine comprising: cutting means for cutting wood;high voltage generation means; connection means for coupling, directlyor indirectly, the high voltage generation means to the cutting means;and detection means for detecting an electrical discharge between thecutting means and a person's body.
 19. The woodworking machine of claim18, where the high voltage generation means includes a Van De Grafgenerator.
 20. The woodworking machine of claim 18, where the detectionmeans includes means for detecting an electrical discharge between thecutting means and a person's body when the person's body is spaced-apartfrom the cutting means.